1. Field of the Invention
The present invention relates to a compound semiconductor switching device, which is an indispensable device for a compound semiconductor switch microwave monolithic integrated circuit (MMIC), and method of fabricating the same and, more specifically, to a transistor of a semiconductor device and method of fabricating the same, which lead to a reduction in insertion loss and increases in isolation and switching speed and are suitable for designing and fabricating a high-frequency control circuit with a high-power low-distortion characteristic.
2. Discussion of Related Art
In general, mobile communication systems, such as mobile phones or wireless LANs, use GHz-band microwaves for communication and include a switching device for switching high-frequency signals as an antenna switch circuit or a transmitter/receiver switch circuit.
The switching device typically employs a field effect transistor (FET) such as a high electron mobility transistor (HEMT) or a metal-semiconductor field effect transistor (MESFET), which are compound semiconductor transistors, because the FET has a good transmission characteristic, a low operating voltage, a small current a simple bias circuit, and an easy implementation of multiport and integrated circuit in a high frequency band.
Also, in a high-frequency switch circuit, it is necessary to minimize insertion loss and improve isolation and switching speed, Above all, a radio-frequency control circuit for a cellular terminal or analog terminal significantly needs a high-power switching device with good linearity.
In order to reduce the insertion loss, conventional techniques selected a method of lowering the resistance of a channel region of a transistor used for a switch circuit by designing the channel region such that its doping concentration or width is as increased as possible.
However, as capacitance caused by a Schottky contact formed between a gate electrode and the channel region increases, a high-frequency input signal is leaked from the Schottky contact, thus degrading the isolation of the transistor.
To improve the isolation of the transistor, conventionally, a method of installing a shunt transistor during a circuit designing process was proposed. However, this method causes the chip size to increase, thus resulting in an increase in the cost of production.
Therefore, in order to fabricate a high-power high-frequency control circuit with good power handling capability using a low-power switching device, circuit design techniques, such as an impedance transformation technique, a stacked FETs method, and an inductor-capacitor (LC) resonant circuit technique, or device structure modification techniques, such as a squeezed-gate FET structure, a two kinds of pinch-off voltage FET structure, and a multigate structure, have conventionally been employed.
However, when the circuit design techniques are used, a chip size increases owing to a transmission line for a λ/4 transformer, a plurality of FETs, and an additional inductor or capacitor adjacent to a switching device, so that the cost of production is on the increase. Further, when the device structure modification techniques are used, the cost of production of chips is raised like in the circuit design techniques because of an additional mask process and an increased distance between a source and a drain.